1. Field of the Invention
The present invention relates to a string chorus generator in an electronic musical instrument that accepts a single audio input signal, applies it to three separate delay lines which are modulated by signals provided from a lookup table by a microprocessor, and provides three audio output signals which are used to produce an ensemble effect.
2. Description of the Prior Art
Apparatus for providing ensemble effects in electrical musical instruments are known in the prior art. Various types of such apparatus which utilize delay lines modulated by low-frequency signals are described in U.S. Pat. No. 3,833,742--van der Kooij; U.S. Pat. No. 3,866,505--Adachi; U.S. Pat. No. 4,038,898--Kniepkamp et al.; U.S. Pat. No. 4,043,243--Peterson; U.S. Pat. No. 4,080,861--Wholahan; U.S. Pat. No. 4,144,790--Suchoff; and U.S. Pat. No. 4,096,778--Dittmar.
Apparatus for providing ensemble effects described in prior art patents utilize a plurality of delay line channels, each of which receives an audio signal which is delay modulated by an analog delay line or shift register clocked by a voltage-controlled oscillator. The output of such shift registers is then filtered and fed to an audio output system. The voltage-controlled oscillators are modulated by a low frequency (sub-audio) signal provided by one or more low frequency oscillators. In some instances, two or more sub-audio frequency signals of different frequencies are summed or superimposed upon one another before being applied to the modulation inputs of voltage-controlled oscillators. Non-linear circuits are used to compensate for the distortion which otherwise occurs to the audio signal being delay modulated as a result of superimposed modulating signals being applied to voltage-controlled oscillators which produce output signals of a frequency directly proportional to the modulating signal. Such non-linear circuits are used either to compensate the modulation control signal applied to the voltage-controlled oscillator or to compensate the clock signals provided by the voltage-controlled oscillator. In other types of prior art apparatus, two or more sub-audio signals are used as modulating signals but the sub-audio signals are applied to separate voltage-controlled oscillators (rather than being superimposed) to minimize distortion to the audio signal. Also, phase shifter networks are used in prior art apparatus in order to provide a plurality of modulation control voltage signals of different phases so that the delay line channels are modulated in predetermined phase relationships with one another rather than in the same phase. Variations are also provided in which, for example, different audio signals are applied to each of the delay line channels, or in which the delay line circuits are connected in series.
Unlike the systems in the prior art for generating ensemble effects, the present invention does not use low-frequency oscillators for controlling the voltage-controlled oscillators which provide clock pulse signals for driving delay lines, the low-frequency oscillators being replaced in the present invention by a lookup table in a microprocessor. Phase-delay networks are also unnecessary in the present invention, the desired phase difference in modulation control voltage signals being provided by the microprocessor when it scans three locations (i.e., one location per modulation control signal) of a common lookup table simultaneously to generate pulse trains corresponding to three independent modulation control signals. Further, in the preferred embodiment of the present invention which utilizes period-proportional voltage-controlled oscillators, non-linear circuits are not needed to compensate for the non-linear relationship between delay and modulation control voltage which results when frequency-proportional voltage-controlled oscillators are utilized. In another embodiment, by properly determining the values stored in the lookup table, modulation control voltage signals are provided by the microprocessor which compensate for the non-linear relationship so that when such signals are applied to frequency-proportional voltage-controlled oscillators, the delay modulation which results is similar to that obtained in the preferred embodiment using uncompensated modulation control voltage signals and period-proportional voltage-controlled oscillators. That is, by properly determining the values stored in the lookup table, the present invention renders unnecessary the phase shifter circuits and non-linear compensating circuits required by prior art apparatus when frequency-proportional voltage-controlled oscillators are used to provide clock signals for analog delay lines.